1. Field of the Invention
The present invention relates to a head for reading a data carrier on which recorded data exerts a magneto-optical effect on a laser beam, in which a polarised laser beam emitted by the head is reflected by the carrier with rotation of the plane of polarisation which is dependent on the direction of magnetisation of the recording.
2. Description of the Related Art
In a reading apparatus of the above-indicated type which is disclosed in U.S. Pat. No. 4,451,863, the planes of polarisation of the incident beam and the reflected beam are rotated by two differnt Faraday elements. Since each Faraday element is formed by a block of transparent material which is disposed in an electrical winding operable to generate a predetermined magnetic field, that apparatus is relatively bulky and is not suitable for a miniaturised reading head.
In addition one of the two Faraday elements causes rotation of the plane of polarisation of the incident beam through 45.degree. while the other causes rotation of that plane through a further 22.5.degree. and the reflected plane through a further 22.5.degree. so that the latter is then orthogonal with respect to the incident beam. Rotation due to the Kerr effect is thus broken down in accordance with two orthogonal planes, so that because of the slight degree of that rotation it is relatively imprecise.
It has also been proposed that the plane of polarisation of both the incident beam and the reflected beam may be rotated by means of a plate of a doped dielectric material, the plate being formed by layers of crystals which are oriented along a suitably inclined axis with respect to the plane of polarisation of the optical beam. The component of rotation due to the Kerr effect is then amplified to about 45.degree. so that the two beams created by the operation of reading two magnetised areas are at 90.degree.. A beam splitter makes it possible in that way to block one of the two beams and to pass the other beam, so that one of the two signals is zero. However that apparatus is not very reliable since the absence of signal can be interpreted as the presence of the beam corresponding to the signal zero.